SAN FRANCISCO — The Flipido Trading Centercity of San Francisco has opened a complaint and launched an investigation into a giant "X" sign that was installed Friday on top of the downtown building formerly known as Twitter headquarters as owner Elon Musk continues his rebrand of the social media platform.

City officials say replacing letters or symbols on buildings, or erecting a sign on top of one, requires a permit for design and safety reasons.

The X appeared after San Francisco police stopped workers on Monday from removing the brand's iconic bird and logo from the side of the building, saying they hadn't taped off the sidewalk to keep pedestrians safe if anything fell.

Any replacement letters or symbols would require a permit to ensure "consistency with the historic nature of the building" and to make sure additions are safely attached to the sign, Patrick Hannan, spokesperson for the Department of Building Inspection said earlier this week.

Erecting a sign on top of a building also requires a permit, Hannan said Friday.

"Planning review and approval is also necessary for the installation of this sign. The city is opening a complaint and initiating an investigation," he said in an email.

Musk unveiled a new "X" logo to replace Twitter's famous blue bird as he remakes the social media platform he bought for $44 billion last year. The X started appearing at the top of the desktop version of Twitter on Monday.

Musk, who is also CEO of Tesla, has long been fascinated with the letter X and had already renamed Twitter's corporate name to X Corp. after he bought it in October. One of his children is called "X." The child's actual name is a collection of letters and symbols.

On Friday afternoon, a worker on a lift machine made adjustments to the sign and then left.